Conventionally, as a method of manufacturing a ball joint of the above type, there has been known one that is disclosed in the Examined Published Japanese Patent Application No. 77886/1993.
As shown in FIG. 17, this method comprises a first step of forming a holder 102 by die-casting using dies 100 having a steel ball 101 inserted therebetween as a core (refer to exploded views a and b), a second step of forming a ball-shank 104 comprised of a shank 103 projection-welded to the steel ball 101 covered with the holder 102 (refer to exploded views c and d); and a third step of applying an external force to the ball-shank 104 or the holder 102 so that a minute gap is formed between the steel ball 101 and the holder 102 closely adhered to each other at the time of forging by the first step, and since the spherical surface of the steel ball 101 is transferred to the holder 102 at the time of casting the latter, a ball joint manufactured by such method as shown in FIG. 18 features that the steel ball 101 and the holder 102 come into sliding contact with each other without rattling thereby making a smooth rotational or oscillating motion.
Further, in the first step of the above-described manufacturing method, the steel ball 101 serving as a core at the time of casting the holder 102 is clamped from on both upper and lower sides thereof by closing the dies 100 in order to keep the steel ball 101 as a core between the dies 100 at the time of casting the holder 102 and the holder 102 is made to have openings 105 and 106 in correspondence to the upper and lower positions of the steel ball 101, respectively. And in the second step, a projection welding operation is performed by using the openings 105 and 106 such that the shank 103 and an electrode 107 for feeding a welding current are brought into contact with the spherical surface of the steel ball 101 through the openings 105 and 106, respectively, and under such condition, a projection welding operation is performed while feeding a predetermined amount of welding current between the steel ball 101 and the shank 103.
Now, where a projection welding operation is performed by bringing the electrode into direct contact with the steel ball according to the above-described conventional method, there is a fear that unless a sufficiently large area of contact between the steel ball and the electrode is secured, a welding resistance to the welding current generates between the electrode and the steel ball which results in melting the steel ball at the position at which the electrode is in contact with the steel ball. Therefore, it is necessary that the opening formed to allow the electrode to pass therethrough has some degree of wideness.
However, such opening for the electrode comes to become coaxial with the shank welded to the steel ball and if the opening is formed sufficiently large, the area of contact between the steel ball and the holder projected along the axis of the shank (refer to FIG. 18) can not but be small so that there has arisen a problem that the capacity of the shank to bear the load acting along the axis thereof, i.e., the axial load, becomes insufficient.
Further, there has also been a problem in order to increase the load-bearing capacity of the shank without reducing the diameter of the opening for the electrode, the diameter of the steel ball has to be made large and the resultant ball joint itself is made large-sized and heavy in weight accordingly.
On the other hand, even when the electrode-passage opening formed in the shank is sufficiently large, if the degree of contact of the electrode itself with respect to the steel ball is not favorable, a welding resistance still generates between the electrode and the steel ball resulting in melting the ball at the contact position of the electrode with the ball. Therefore, in the case of the above-described conventional manufacturing method, a copper plate electrode is formed to have a spherical surface after the example of the spherical surface of the ball but such arrangement has had the disadvantage that since the configuration of the electrode changes gradually while repeating the projection welding, it is necessary to reform the electrode every time when a predetermined number of welding operations is performed and so the production control becomes quite troublesome.
Further, the above-described conventional manufacturing method has had the problems that it mainly uses a zinc-based die-casting alloy (for example, a zinc-aluminum-copper type die-casting alloy) as a holder forming material but although such die-casting zinc alloy has an excellent degree of mechanical strength such as a tensile strength, hardness, tenacity and etc., since it lacks anti-corrosiveness and has a large specific gravity, the holder tends to become corroded and can not but become heavy in weight, the location of using a link mechanism formed by using a ball joint manufactured by the method is limited and the reduction of weight of the link mechanism is hindered.
On the other hand, there may be given an aluminum-based material as a die-casting alloy having a small specific gravity but such die-casting aluminum alloy in its state of being die-cast is inferior to the above-described die-casting zinc alloy with respect to its tensile strength and hardness and a ball joint manufactured by using this alloy can not be used at the structural portion of which a mechanical strength is required, for example, the portion like the tie rod at the suspension of an automobile.